An example of a conventional solid-state image pickup device is shown in FIG. 4. FIG. 4 is a top view of a part of a light detecting region in the solid-state image pickup device, which may be a charge storage device of the interline transfer type, showing the basic structure of the solid-state image pickup device. A plurality of n-type layers are formed in matrix arrangement in a p-type silicon layer (p-well) formed on the surface of a semiconductor substrate, thus providing a plurality of light detecting elements (photodiodes) Pd.sub.11 Pd.sub.12, . . . , Pd.sub.21, Pd.sub.22, . . . A plurality of charge transfer channels L.sub.1, L.sub.2, L.sub.3, . . . are formed between the photodiodes in columns to transfer signal charges in a vertical direction. The regions (shown surrounded by dotted lines and shaded by oblique lines in the drawing) except for those of the photodiodes and the charge transfer channels L.sub.1, L.sub.2, L.sub.3, . . . are channel stop regions.
A plurality of transfer electrodes G.sub.1, G.sub.2, G.sub.3, G.sub.4, . . . formed of polysilicon layers extend horizontally over the charge transfer channel. With four transfer electrodes as one group, clock signals .phi..sub.1, .phi..sub.2, .phi..sub.3, and .phi..sub.4 generated according to a four-phase drive system are applied to the transfer electrodes (G). More specifically, two transfer electrodes G.sub.1 and G.sub.2 are formed for the line of photodiodes Pd.sub.11, Pd.sub.12, . . . , two transfer electrodes G.sub.3 and G.sub.4 are formed for the line of photodiodes Pd.sub.21, Pd.sub.22, . . . , and so on for the remaining lines (not shown). Potential wells are formed in the charge transfer channels L.sub.1, L.sub.2, L.sub.3, . . . according to the variations in voltage of the clock signals .phi..sub.1, .phi..sub.2, .phi..sub.3 and .phi..sub.4, so that signal charges generated by the photodiodes are transferred in the output direction Y. The photodiodes are connected to the charge transfer channels L.sub.1, L.sub.2, L.sub.3, . . . through transfer gates Tg.sub.11, Tg.sub.12, . . . , and are turned on by application of a predetermined high voltage to the transfer electrodes G.sub.2, G.sub.4, . . . .
The charge transfer operation according to the four-phase drive system will be described with reference to FIGS. 5 and 6. FIG. 5 is a sectional view taken along line X--X in FIG. 4, showing relationships between the transfer electrodes G.sub.1, G.sub.2, G.sub.3, G.sub.4, . . . and the potential wells. The photodiodes (Pd) and the transfer electrodes (G) are combined in such a manner that, for instance, the photodiode group Pd.sub.12 and the transfer electrodes G.sub.1 and G.sub.2 are arranged in an odd-numbered row, and the photodiode group Pd.sub.22 and the transfer electrodes G.sub.3 and G.sub.4 are in an even-numbered row, in which case a set of signal charges can be transferred with four transfer electrodes as one group.
The transfer operation will be described in detail with reference to the potential profile shown in FIG. 6. When, for instance, at the time instant t.sub.0, the voltage levels of the signals .phi..sub.1, .phi..sub.2, .phi..sub.3 and .phi..sub.4 are set to "L", "H", "H" and "H", respectively, the signal charges generated by the photodiodes Pd.sub.12 and Pd.sub.22 in the odd-numbered and even-numbered rows can be transferred to the potential well as shown in FIG. 6. When, at the next time instant t.sub.1, the voltage levels of the clock signals .phi..sub.1, .phi..sub.2, .phi..sub.3 and .phi..sub.4 are set to "L", "L", "H" and "H", respectively, a potential barrier is formed under the transfer electrode to which the clock signal .phi..sub.2 is applied, and therefore the signal charges are shifted in the transfer direction. In succession, the voltage levels of the clock signals .phi..sub.1, .phi..sub.2, .phi..sub.3 and .phi..sub.4 are set "H", "H", "L" and "H", respectively, at the time instant t.sub.4 to transfer the signal charges. Thus, the transfer operation is achieved using the above-described four phases of the potential profile.
However, the above-described solid-state image pickup device suffers from the following problems: In the case where the solid-state image pickup device is applied to an image pickup device such as an electronic camera, it is essential to determine various photographing factors such as exposure, focus and white balance before an actual photographing operation. The time required for this process before the photographing operation is relatively long since recent image pickups employ a high picture element density. Therefore, the operator may consider that the camera is relatively slow in operation.
For instance, in the adjustment of white balance in an electronic camera employing a TTL system, first, incident light from the object is converted into a video signal with a solid-state image pickup device, then a luminance signal and color signals are derived from the video signal. Next, the amplification factors of amplifiers for amplifying these color signals are adjusted by feeding back the color signals so that they correspond to the reference "white". Similarly, lengthy processing must be carried out for exposure setting and focussing. Only when all of these operations have been completed can photographing operations be carried out in the ordinary manner.
To achieve the above-described adjustments, all signal charges generated by all the light detecting elements in the matrix are read, and these signals are processed. In order to accomplish the adjustment with high accuracy, the signal reading operation must be carried out at least several times. Therefore, the time required for all processes preparatory to the actual photographing operation is considerably long. Concretely stated, the time required for reading signal charges for one frame is generally 1/30 second, and accordingly for ten repetitions the total time is 1/3 second. Since the signal processing time must be added to this time to determine the total picture-taking interval, which time may even be longer, the total picture-taking interval is so long as to adversely affect the operability and performance of the camera.